Hello, I have questions about Age-Period-Cohort models in SAS. I used proc genmod for age-specific fertility rates (Births/Exposure Female Population). I need to identify what effects are more important and how they changed fertility rates.The results are risk ratios. Explaining variables are mother’s age, Period (4 periods of 5 years length and 1 period of 4 years) and Cohort. Because of identification problem I made a constraint on Period (and chose that period 2012=reference period 2007.5). I overwrote it in the dataset (you can see it in Excel). The formula of reggression is: ln(r ap) = intercept + α a + β p + γ c + ε if I use Poisson distribution. But I cannot use Poisson distribution because of overdispersion. So I tried negative binomial distribution. The formula for binomial distribution should then be ln(r ap /(1-r ap) ) = intercept + α a + β p + γ c + ε but I really don't know if and how I should set it in the SAS code. Or is it adjusted? proc genmod data=genmod.input; class Age (ref='32.5') Period (ref='2007.5') Cohort (ref='1975') /param=ref; model Birth = Age Period Cohort / link=log dist=nb offset=log_Exposure type3; run; If I work on the assumption that the code is correct (I mean in part model Birth and offset=) I have another problem. Even though I use negative binomial distribution, data are also overdispersed when all variables are used. Could you please advise me? Thank you very much in advance. Anna K.
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